1. Field of the Invention
The present invention relates to a suspension system for a vehicle, such as tractor.
2. Description of the Related Art
As the prior art of a suspension system for a vehicle, a technique of EP0761481B1 has been known in which: to a vehicle body is attached an engine whose rear portion is connected to a transmission case and whose front portion has a front frame projecting frontward; a front portion of the vehicle body is configured to support a rear portion of a swing frame swingable about a support shaft extending in a lateral direction; the swing frame is configured to support a front axle case (having suspended front wheels) swingable about a center shaft extending in a front-rear direction; and on the front frame is provided right and left suspension cylinders for the front wheels, which absorb, through the swing frame, impacts transmitted from the front wheels to the front axle case.
With respect to the swing frame, a front portion may rock (or oscillate) laterally with the support shaft provided in the rear portion serving as a center of oscillation, due to loads on the right and left front wheels. In order to prevent the durability of the support shaft from reducing, which may otherwise be caused by the oscillation load, a lateral oscillation regulation unit is provided.
Generally in a four-wheel vehicle, a braking force and an accelerating force acting on the front axle case are shared by right and left lower links, right and left upper links and the like, and a load in the lateral direction is shared by a lateral rod. As the lateral oscillation regulation unit, a lateral rod has been used, whose one end is pivotally supported by the vehicle body, and whose the other end is pivotally supported by the front axle case (see, for example, Automotive Technology Handbook, Designing (Chassis), p 8-9, published by Society of Automotive Engineers of Japan, Inc.). In the case where the lateral rod connecting the vehicle body and the front axle case is used for regulating a lateral oscillation of the swing frame, when the front axle case vertically swings, a front axle case-end of the lateral rod undergoes an arc movement with a vehicle body side-end serving as a movement center. Therefore, the front portion of the swing frame supports a load in the lateral direction while rocking in the lateral direction. As a result, it becomes difficult to prevent the support shaft in the rear portion of the swing frame from being affected by overload.